mkdir /zzz mount /dev/sda3 /zzz mount -t proc none /zzz/proc mount -o bind /dev /zzz/dev chroot /zzz /bin/bash grub find /boot/grub/stage1 root (hd0,2) setup (hd0)
For /boot/grub/menu.lst, a minimal config would be

default Linux timeout 2 title Linux root (hd0,2) kernel=/boot/vmlinuz root=/dev/sda3 ro
also check /zzz/etc/fstab.